1. Field of the Invention
The present invention relates to a novel novolak thermosetting resin, in particular, a novolak resin to be used in a photoresist, a process for the preparation of said novolak resin and a positive resist composition which comprises said novolak resin and is excellent in sensitivity, heat resistance and film thickness retention.
2. Description of the Related Art
Since a phenol resin which is prepared by a reaction between a phenol compound and a carbonyl compound is excellent in heat resistance, it is used as a thermosetting resin in a wide variety of industrial fields such as a woodworking industry and an electric industry. In particular, in recent years, it is used as a pattern-forming material (resist) in the formation of minute patterns of an integrated circuit in a semiconductor field.
For example, a resist comprising a novolak resin which is prepared by an addition condensation of an aldehyde with at least one of m-cresol and p-cresol is generally used in the pattern formation using the i-line or the g-line as a light source, but its heat resistance is still unsatisfactory. Further, the resist comprising the above novolak resin is not necessarily satisfactory as a pattern-forming material for 64M DRAM in the next generation and 256M DRAM in the next but one generation. In particular, for the use of an excimer laser lithography which uses an excimer laser as a light source, its transparency is insufficient, and a submicron order pattern has not been formed.
A radiation-sensitive resist composition containing a compound having a quinone diazide group finds use as a positive resist, because upon exposure to light having a wavelength of not longer than 500 nm, the quinone diazide group decomposes to form a carboxyl group whereby the originally alkali-insoluble composition becomes alkali-soluble. The positive resist composition has much better resolution than a negative resist composition and is used in the production of integrated circuits such as IC or LSI.
Recently, particularly in the production of integrated circuits, miniaturization has proceeded as the integration level has increased, which results in demands for formation of patterns of submicron order. According to conventional processes for the production of integrated circuits, light exposure is accomplished by placing a mask in intimate contact to a substrate, e.g. a silicon wafer. It is said that this process cannot make patterns thinner than 2 .mu.m. Instead of such conventional processes, the reduction projection in exposure system attracts attention. According to this new system, a pattern of a master mask (reticle) is projected on the substrate with reduction by a lens system, whereby exposure is accomplished. This system realizes a resolving power of submicron.
One of the serious problems in this system is low throughput. Namely, in this system, the total exposure time to expose a wafer is very long because of divided and repeated light exposure unlike a batch light exposure system which is employed in the conventional mask contact printing methods.
To solve this problem, not only an improvement in the apparatus but also an increase in sensitivity of the resist to be used are important.
To increase the sensitivity, a molecular weight of an alkali-soluble resin is decreased. However, the decrease of the alkali-soluble resin molecular weight deteriorates heat resistance.
One of other measures to increase the sensitivity of the resist is selection of a monomer which is added to the alkali-soluble resin so as to increase a dissolving rate of the resin in an alkaline developing solution. For example, in case of a novolak resin, when a ratio of a monomer which is highly soluble in the alkali developing solution such as m-cresol or phenol, the dissolving rate of the resin in the developing solution can be increased.
By the above measure, though the sensitivity of the photoresist is increased without deterioration of heat resistance and also without decreasing the molecular weight of the alkali-soluble resin, film thickness retention is deteriorated, which results in decrease of resolution.
In general, it is impossible to improve the sensitivity with maintaining heat resistance and the film thickness retention, and vice versa.